A nitride semiconductor has features such as a high saturation electron speed, a wide band gap, etc. Thus, it is considered to apply the nitride semiconductor to semiconductor devices having a high withstand voltage and a high output. For example, the bad gap of GaN, which is a nitride semiconductor, is 3.4 eV, which is higher than the band gap of Si (1.1 eV) and the band gap of GaAs (1.4 eV). Thus, GaN has a high breakdown electric field strength. Accordingly, the nitride semiconductor such as GaN or the like is extremely hopeful as a material to fabricate a power supply semiconductor device providing a high-voltage operation and a high-output.
As a semiconductor device using a nitride semiconductor, there are many reports with respect to a filed effect transistor, particularly, a high electron mobility transistor (HEMT). For example, from among GaN-HEMTs, an HEMT made of AlGaN/GaN attracts attention wherein GaN is used as an electron transit layer and AlGaN is used as an electron supply layer. In the HEMT made of AlGaN/GaN, a strain is generated in AlGaN due to a difference in lattice constant between GaN and AlGaN. Thereby, a highly concentrated two-dimensional electron gas (2DEG) can be obtained due to a piezoelectric polarization caused by such a strain and an intrinsic polarization difference. Thus, the AlGaN/GaN-HEMT is hopeful as a high-efficiency switch device and a high withstand voltage power device for electric vehicle. Additionally, from a view point of circuit design and safety, it is desired to materialize a nitride semiconductor transistor having a normally off characteristic.
The following patent documents disclose a background art.
Patent Document 1: Japanese Laid-Open Patent Application No. 2012-151422
Patent Document 2: Japanese Laid-Open Patent Application No. 2012-9630
Patent Document 3: Japanese Laid-Open Patent Application No. 2008-124373
In the meantime, in order to obtain a low cost and high quality nitride semiconductor transistor, it is necessary to cause a nitride semiconductor to be formed by high-quality epitaxial growth on a low cost silicon (Si) substrate having a large diameter. However, silicon and a nitride semiconductor such as GaN are different, from each other not only in their lattice constant but also in their coefficient of thermal expansion. Thus, in order to cause a high-quality GaN film to grow on a silicon substrate, an appropriately designed superlattice buffer layer is formed on the silicon substrate and a GaN film is formed on the superlattice buffer layer. The superlattice buffer layer can be formed by, for example, an AlN film and an AlGaN film, which are alternately laminated to form a lamination of a plurality of layers having a periodic structure. An electron transit layer and an electron supply layer are laminated on such a superlattice buffer layer.
In a nitride semiconductor transistor having the above-mentioned structure, a high-voltage is applied to a drain electrode while the transistor is in operation. If an insulation property of the superlattice buffer layer is low, there may be a case where a leak current flows in a vertical direction from the electron supply layer toward the silicon substrate via the superlattice buffer layer.
Thus, it is desirous to materialize a nitride semiconductor device formed on a silicon substrate, the semiconductor device having an insulating superlattice buffer layer and having a small leak current flowing in a vertical direction.